In the field of the broadcasting of contents as defined above and in particular of television contents with conditional access destined to mobile receivers, known with the acronym DVB-H (Digital Video Broadcast-Handheld), the contents are sent in a cyclic way in the form of packets (known as Time Slices). The packets correspond to services. A service can be defined as one or more digital streams forming a subset of the global digital stream. This subset shares a common identifier so as to be able to create an event starting from this identifier.
The rights and other controls are sent in the form of authorization messages also in the form of packets. These packets of authorization messages are considered as a specific service and are therefore sent in the form of service data.
In a broadcast environment, each authorization message is repetitively broadcast in order to be able to effectively reach its receiver. When the number of subscribers or users is high, the number of messages to be broadcast is likewise. The packet of each cycle containing these messages can occupy a considerable broadband. In practice, the number of authorization messages intended for each receiver is relatively low. Nonetheless, all the receivers receive the packet containing the authorization messages and process them in order to establish if these messages relate to them or not.
When using mobile receivers working with batteries, the energy consumption problem is particularly important. Actually, in order to ensure the receiver's maximum autonomy, it is useful to try to reduce the energy consumption to a minimum. For this reason, it is therefore essential to process as little useless information as possible.
In the prior art systems, the authorization messages, containing in particular access rights to events or keys which allow for the decryption of encrypted information, are sent to all the receivers which have access to the concerned services. Among this set of receivers, each authorization message is actually intended for a low number of receivers. These must however process these authorization messages, in particular they must receive and decrypt them if necessary, before being able to establish whether the receivers are really concerned or not.
This implies that the receiver must be in active mode at the moment of the reception of the packet of authorization messages and that it must consume energy for receiving and processing messages that maybe do not concern it. Energy is therefore often wasted, which can considerably reduce the autonomy of the mobile receiver.
The application EP 0 975 109 describes a method for broadcasting digital content to receivers, in which these receivers are started only when a content is intended for them. To realize this, the method involves the preliminary sending of information tables to each receiver. These information tables involve an indication of a type of transmitted information, an identifier of the receivers to which the information is intended for as well as a time interval during which the information will be broadcast.
At the reception of an information table, the receiver determines which group it belongs to and memorizes the time ranges corresponding to the broadcasting of information related to it. Then it sets itself up in standby mode and sets itself up in receiving mode only a short time before information relating to it is broadcast.
The concerned receivers are in inactive state and set themselves up in receiving state some seconds before the information intended for them is sent. This information is sent for each group of receiver for about ten minutes.
In a system using portable decoders, a duration of some seconds corresponds to a complete cycle of messages being sent. Consequently, by using the system described in the EP 0 975 109, no energy saving is possible. This method is therefore not compatible with the broadcasting on mobile receivers.
Moreover, in order for a receiver to be able to have access to the authorization messages relating to it, it is necessary for it to receive the corresponding table. In an application using in particular mobile receivers, this means that this table must be sent very frequently so that a user starting its unit in a certain moment would not have to wait for a long time before being able to receive the corresponding messages. It so follows that the receiver must be started up to obtain the table concerning it, which is in contradiction with the aim of the invention, that is, to reduce energy consumption.